The present invention concerns a device to produce a modulated electric field for an electrode. This applies in particular to flat field emission screens.
The devices for visualization by cathodoluminescence excited by field emission are well known. Such a device comprises a cathode arranged facing an anode. The cathode is a flat structure emitting electrons and the anode is another flat structure covered with a luminescent film. These structures are separated by a space in which a vacuum is created.
The cathode can be a source of microtips or a source with a low threshold field emissive material (the threshold field being the electric field needed to extract electrons from a material), for example nanostructures or carbon. The sources with an emissive material used in screen devices are usually shown in two formsxe2x80x94a diode type structure or a triode type structure.
FIG. 1 shows in a transversal cross-section view, a flat field emission screen operating according to a diode type structure. The cathode 1 is made up of a plate of insulating material 3 supporting parallel metallic tracks 4 and covered with layers of an emissive material 5. The anode 2 is an insulating and transparent plate 6, for example in glass, supporting parallel conductor tracks 7 and at right angles to the cathode tracks 4. The tracks 7 are made by the etching of a layer of a transparent conducting material such as tin and indium mixed oxide (ITO). The tracks 7 are covered with films of phosphor 8.
The cathode plate and anode plate are placed facing one another, the tracks being opposite to make up a matrix structure. The crossing of the track networks forms image elements or pixels. By applying an adequate potential difference between one track 4 of the cathode and one track 7 of the anode, an emission of electrons occurs on the zone of the track 4 corresponding to the considered pixel, and the zone of the phosphor 8 facing is excited. A complete image can be obtained on the screen by successively supplying each line of the screen and by sweeping.
So that electrode emission occurs, an emissive material with low threshold field such as carbon needs a minimum electric field of several V/xcexcm between an anode track and a facing cathode track. If the space between these tracks is 1 mm, a potential difference of several kV must therefore be applied, usually between 5,000 and 10,000 V. This leads to two main problems. The first is the resistance in voltagexe2x80x94there is danger of breakdown between anode and cathode and above all between two adjacent tracks. The second problem results from the need to switch a voltage of several kV when sweeping the screen. This problem can be resolved by reducing the space between anode and cathode which facilitates reducing in the same way the potential difference between them while maintaining the same electric field. The disadvantage of this solution is that this decrease in potential causes a decrease in the output of the phosphors and less brilliance in the screen.
The triode type structure has been suggested in order to try and remedy these problems. FIG. 2 shows in transversal cross-section a flat field emission screen implementing such a structure. The cathode 11 is made up of a glass plate 13 supporting parallel metallic tracks 14 and covered with layers 15 of an emissive material, carbon for example.
The tracks 14 are placed on the bottom of trenches etched in a layer of insulating material 10, the layer 10 being covered with a metal layer 19 serving as extracting gate. The anode 12 can be made up of a transparent plate 16 with for example a transparent and conductive film 17 covered by a film of luminescent material 18.
An emission of electrons by the emissive material can be obtained by applying, between the extraction gate 19 and track 14, a potential difference so that the resulting electric field on the emissive material is greater than the threshold field of this material, usually several V/xcexcm. As the distance separating the extraction gate from the tracks is very much smaller than the distance separating the anode from the cathode, the potential difference to be applied is reduced in the same way.
As the lines of electric field go from tracks 14 to the extraction gate 19, a large part of the electrons emitted is going to be trapped by the gate. The triode type structure therefore has the disadvantage resulting from the fact that very few of the electrons emitted reach the phosphor layer.
Such a visualization device of triode type structure therefore enables avoiding the risk of electric breakdown and the problems of high voltage switching. However, these improvements are obtained to the detriment of electron density emitted which reach the luminophore or phospor layer. Moreover, this type of structure needs the realization of a deposit of emissive material solely on the bottom of trenches which presents considerable difficulties.
The present invention provides for solving the problems set forth above. The solution consists in applying a modulation electric field near to an electrode in the vicinity of which one wishes to obtain an electric field of specified value. Depending on the case, the modulation electric field will have the effect of decreasing or increasing the value of the electric field in the vicinity of the electrode in question.
The first object of the invention concerns a device which permits producing an electric field between a first and a second electrode, comprising:
means for applying a potential difference between these two electrodes, allowing to obtain, if this potential difference is applied alone, a predetermined value of electric field in the vicinity of the first electrode,
means forming modulation electrode located near the first electrode, either on the same plane or so that the first electrode is inserted between the second electrode and said means forming modulation electrode,
control means for applying a potential difference between the means forming modulation electrode and the first electrode in order to obtain through the contribution of said potential differences another predetermined value of electric field in said vicinity of the first electrode.
In a first case, the means for applying a potential difference between the first and the second electrode and the control means, supply potential differences such that the value of the electric field in said vicinity of the first electrode is greater than the value which would be due to the potential difference alone between the first and the second electrode.
In a second case, the means for applying a potential difference between the first and the second electrode and the control means, supply potential differences so that the value of the electric field in said vicinity of the first electrode is lower than the value which would be due to the potential difference alone between the first and the second electrode.
Conveniently, the first and the second electrode and the means forming modulation electrode are arranged parallel.
The means forming modulation electrode can comprise two electrodes surrounding the first electrode.
If the first electrode is inserted between the second electrode and the means forming modulation electrode, the means forming modulation electrode can be made up by a single electrode.
The second object of the invention concerns a process for producing an electric field between a first and a second electrode comprising:
the application of a potential difference between the first and the second electrode so as to obtain, if this potential difference was applied alone, a predetermined value of the electric field in the vicinity of the first electrode,
the application of a potential difference between the first electrode and means forming modulation electrode and located near to the first electrode, either in the same plane or so that the first electrode is inserted between the second electrode and said means forming modulation electrode, in order to obtain in association with the electric field due to the application of the potential difference between the first the second electrode, another predetermined value of electric field.
In a first case, the application of the potential difference between the first and the second electrode is such that if this potential difference was applied alone, the electric field in said vicinity of the first electrode would be greater than said other predetermined value.
In a second case, the application of the potential difference between the first and the second electrode is such that if this potential difference was applied alone, the electric field in said vicinity of the first electrode would be lower than said other predetermined value.
A third object of the invention concerns a field emission screen comprising an anode plate and a cathode plate facing one another, the anode plate comprising on its internal surface of the screen at least one electrode supporting phosphor means, the cathode plate comprising on its internal surface of the screen at least one electrode emitting electrons at least partially facing the anode electrode, this cathode electrode becoming emitter of electrons when the electric field in its vicinity exceeds a threshold value, the screen also comprising application means for a potential difference between said anode electrode and said cathode electrode, characterized in that the screen further comprises means forming modulation electrode located in the vicinity of the cathode electrode, either on the same plane or so that the cathode electrode is inserted between the anode electrode and said means forming modulation electrode, the screen also comprising control means for applying a potential difference between the cathode electrode and the means forming modulation electrode, the means for applying potential differences is such it provides for obtaining in said vicinity of the cathode electrode a predetermined value of electric field resulting from the contribution of said potential differences, said predetermined value being as one wishes either lower than said threshold value, or greater than said threshold value.
In a first case, the means for applying a potential difference between said anode electrode and said cathode electrode is such that, in the absence of a potential difference applied between the cathode electrode and the means forming modulation electrode, said predetermined value of electric field is lower than said threshold value.
In a second case, the mains for applying a potential difference between said anode electrode and said cathode electrode is such that, in the absence of a potential difference applied between the cathode electrode and the means forming modulation electrode, said predetermined value of electric field is greater than said threshold value.
The means forming modulation electrode can comprise two electrodes surrounding the cathode electrode.
If the cathode electrode is located between the anode electrode and the means forming modulation electrode, the means forming modulation electrode can be made up of a single electrode.
Advantageously, the cathode electrode and the means forming modulation electrode are separated by a layer of insulating material.
Preferably, the cathode electrode comprises a conductive part on which is deposited a layer of emissive material. This layer of emissive material can be separated from the conductive part by a resistive film. The layer of emissive material need only cover part of the resistive film. The emissive material can be a material deposited on the resistive film by a catalyst material deposited on the resistive film and on which the emissive material settles preferentially.
The display screen is conveniently of the matrix type, the crossing of lines and columns defining pixels.
According to a preferred arrangement, the anode plate comprises a common electrode with phosphor means, the cathode plate comprises a plate supporting conductor lines constituting the means forming modulation electrode, covered with a layer of dielectric material, the layer of dielectric material supporting the conductive columns, the lines and columns forming a matrix arrangement connected to addressing means and defining pixels, the conductive columns having an emissive material. Each pixel can correspond to the crossing of a line and several column conductors.
According to a specific arrangement, the conductive lines comprise windows facing the conductor columns, the emissive material supported by the conductor columns being only present on the areas of the conductor columns corresponding to the windows.
A fourth object of the invention concerns a process for the use of a field emission screen comprising at least one anode electrode and at least one cathode electrode facing, the cathode electrode comprising an emissive material emitting electrons when the electric field in the vicinity of the cathode electrode exceeds a threshold value, characterized in that, in order to obtain an emission of electrons on the part of the emissive material, it comprises:
the application of a potential difference between the anode electrode and the cathode electrode so as to obtain in the vicinity of the cathode electrode, if this potential difference was applied alone, an electric field of value lower than said threshold value,
the application of a potential difference between the cathode electrode and the means forming modulation electrode located near the cathode electrode, either in the same plane or so that the cathode electrode is inserted between the anode electrode and said means forming modulation electrode, so as to obtain in said vicinity of the cathode electrode, in association with the electric field due to the application of the potential difference between the anode and cathode electrodes, an electric field value greater than said threshold value.
A fifth object of the invention concerns a process for the use of a field emission display screen comprising at least one anode electrode and at least one cathode electrode facing, the cathode electrode comprising an emissive material emitting electrons when the electric field in the vicinity of the cathode electrode exceeds a threshold value, characterized in that, in order to avoid an emission of electrons from the emissive material, it comprises:
the application of a potential difference between the anode electrode and the cathode electrode so as to obtain in the vicinity of the cathode electrode, if this potential difference was applied alone, an electric field greater in value than said threshold value,
the application of a potential difference between the cathode electrode and the means forming modulation electrode located in the vicinity of the cathode electrode, either in the same plane or so that the cathode electrode is inserted between the anode electrode and said means forming modulation electrode, so as to obtain in said vicinity of the cathode electrode, in association with the electric field due to the application of the potential difference between the anode and cathode electrodes, an electric field value lower than said threshold value.